Coalbed Methane (CBM) is a natural gas formed by geological processes in coal seams and consists predominantly of methane, the major chemical component in natural gas. CBM is an all in one natural gas resource as it serves as the source, reservoir and trap for a vast amount of potential natural gas. Typically, CBM can be found unexploited at very shallow depths, and because methane is stored in coal by a different means than conventional gas, more gas per unit volume can be recovered at these shallow depths.
Various methods have been utilized by the energy industry to extract CBM from a subterranean formation. In most instances well bores are drilled to penetrate the hydrocarbon-containing portions of the subterranean formation into sections of the subterranean formation commonly referred to as “production intervals.” A subterranean formation penetrated by a well bore may have multiple production intervals at various depths in the well bore.
Generally, after a well bore has been drilled to a desired depth, completion operations may be performed, usually involving the insertion and cementing of steel casing into the well bore. To be able to produce hydrocarbons from the coal seam, one or more perforations are normally created to penetrate through the casing and cement, and into the production interval. Generally, the produced hydrocarbons flow from the production intervals, through the perforations that connect the production intervals with the well bore, into the well bore, and ultimately to the surface. At some point during completion operations, the production interval may be “stimulated” to enhance hydrocarbon production.
Numerous methods have been developed and used successfully for stimulating the production of CBM from coal seams. Typical stimulation operations may involve hydraulic fracturing, acidizing, or fracture acidizing.
Hydraulic fracturing stimulation typically includes injecting or pumping a viscous fracturing fluid into a portion of the subterranean formation at a rate and pressure such that fractures are formed or enhanced into the portion of the subterranean formation. The incident pressure causes the formation to crack which allows the fracturing fluid to enter and extend the crack further into the formation. The fractures tend to propagate as vertical and/or horizontal cracks located radially outward from the well bore.
In such treatments, once the hydraulic pressure is released, the fractures formed will tend to close back onto themselves, possibly cutting off any hydrocarbon flow. To prevent this closure, oftentimes a particulate material, known as “proppant,” is placed in the fractures by suspending them in the injected fracturing fluid during at least a portion of the fracturing operation. The proppant is carried into the newly created fractures and deposited therein such that when the hydraulic pressure is released the proppant acts to prevent the fracture from fully closing, and thus, aids in forming conductive channels through which fluids may flow into the well bore. The proppant, commonly a sieved round sand, is used since it is higher in permeability than the surrounding formation. The propped hydraulic fracture, therefore, becomes a highly permeable conduit through which the formation fluids can be produced back to the well.
In some applications, hydraulic fracturing stages are immediately followed by the injection of an acidizing solution. The acid serves several functions including flowing above the fracture fluid and proppant deposited in the lower portion of a vertical fracture thus having a tendency to widen and vertically extend the upper portion of a fracture. Acidizing may also initiate new fractures and clean the well bore and fracture faces by dissolving any precipitates or contaminants due to drilling or completion fluids or cement which may be present at or adjacent the well bore or fracture faces.
Although there are several fracturing and stimulation methods already in use, it remains desirable to find improved methods for fracturing subterranean coal seams. In particular, it is desirable to find methods that may introduce different fracturing fluids having diverse chemical properties and methods that reduce or eliminate the need for proppants. By doing so, significant time and expense savings may be accrued by not having to provide a typical fracture fluid and propping agent.